In order to build an integrated circuit, many active devices need to be fabricated on a single substrate. The current practice in semiconductor manufacturing is to use thin film fabrication techniques. A large variety of materials can be deposited using thin films, including metals, semiconductors, insulators, and the like. The composition and uniformity of these thin layers must be strictly controlled to facilitate etching of submicron features. The surface of the substrate, most often a wafer, must be planarized in some way to prevent the surface topography from becoming increasingly rough with each added thin film level. Every layer deposited on the top surface of the wafer that possesses irregularities and variations in its uniformity has an adverse affect during all subsequent processing steps that the wafer undergoes. Uniformity of the layers is a critical factor in semiconductor wafer production. The formation of such films is accomplished by a large variety of techniques.
Chemical vapor deposition (CVD) processes are often selected over competing deposition techniques because they offer numerous advantages, including the abilities of CVD to deposit films from a wide variety of chemical compositions and provide improved conformability.
In general, a CVD process includes the following steps: a selected composition and flow rate of gases are dispatched into a reaction chamber; the gases move to the substrate surface; the constituents making up those gasses are adsorbed on the substrate surface; the constituents undergo migration and film-forming chemical reactions; and the by-products of the reactions are desorbed from the surface and conveyed away from the surface.
Plasma enhanced CVD (PECVD) uses a plasma or glow discharge with a gas to create reactive species of the gasses introduced into the reaction chamber. This allows the substrate to remain at a lower temperature than in other CVD processes. A lower substrate temperature is the major advantage of PECVD and provides film deposition methods for substrates that do not have the thermal stability necessary for other processes that require higher temperature conditions. In addition, PECVD can enhance the deposition rate, when compared to thermal reactions alone, and produce films of unique compositions and properties.
As thin films cover changes in elevation that occur on the surface of the underlying substrate, they often suffer unwanted deviations from the ideal conformality, such as thinning or cracking. A measure of how well a film maintains its nominal thickness is referred to as the step coverage of the film. The height of the step and the aspect-ratio (the height-to-spacing ratio of two adjacent steps) of a feature being covered determine the expected step coverage.
The semiconductor industry's continuing drive towards closer and smaller device geometries has placed an increased demand for cost-effective solutions for the problem of higher step coverage and planarization. New plasma sources are being developed to extend to the sub-0.5 micron level of processing necessary for the more rigorous device geometries. CVD processes have been developed for some metals, for example titanium and titanium nitride, both of which can be put to use in 0.35 and 0.25 micron devices, as well as smaller devices. This is especially useful in processes taking place toward the end of the fabrication procedure, also known as back-end-of-the-line (BEOL) processes. At this point, layers have already been deposited and doped, yet the semiconductor device must still undergo further fabrication. For example, interconnects may still have to be formed. As a result, the BEOL fabrication processes must be done at low temperatures (&lt;450.degree. C.) to protect the integrity of these previously deposited layers and to ensure that dopants do not diffuse excessively. Thus, BEOL processes are typically based on PECVD, which, as described above, can be achieved at low temperatures.
These low temperature, high aspect ratio coverage PECVD process requirements are being met with low pressure, high density plasma (HDP) based processes. To achieve the good step coverage and gap fill desired, HDP CVD systems are run at a high flow rate to achieve adequate deposition. At the same time HDP CVD process pressures need to be relatively low for the plasma to operate at high densities.
To deposit conductive or metal films using high density PECVD, it is preferred that the plasma be generated using inductive coupling. The deposition of metal thin films in an inductively coupled high density plasma reactor is desirable because of the advantages it provides, including: lower processing temperatures and higher step coverage, as discussed above, as well as shorter deposition times and denser films.
Plasma deposition or etching processes using chemical reactions which are not very favorable thermodynamically result in very low deposition or etching rates. In many cases simply increasing the process pressure is not feasible as described above for the low pressure and high density PECVD processes. Increasing the pressure would most likely lead to degradation in step coverage or inclusion of impurities into the film. In such cases, it is desirable to increase the ionization efficiency of the reactants in the plasma.
One method to improve coverage is to increase the acceleration of the ions from the discharge toward their surfaces. The impinging ions transfer energy to surface atoms, and cause them to be transported to the sidewalls of structures on or formed from the substrate, where they accumulate and locally increase film thickness. There is a constant need in the art, however, for alternative methods of improving providing good deposition in general and in PECVD processes in particular.